In a fluid driving device such as a blower or pump, a motor connects with an impeller to drive the impeller to rotate during operation. At startup of a motor with small and greatly-fluctuating startup torque, such as a single phase synchronous motor or single phase direct current brushless motor, because the impeller has a large rotational inertia and startup load torque, vibrations may easily occur during startup of the motor; or even worse, the motor startup may failure.
In a typical method of starting the single phase motor under load, a friction startup device is used to allow the motor to rotate firstly, which in turn drives the impeller to rotate progressively. Currently, the friction startup device consists of multiple arcuate plates and an annular spring. Multiple arcuate plates are disposed on the impeller and are located on a same imaginary circle. The annular spring surrounds outer sides of the multiple arcuate plates. An end portion of a rotary shaft of the motor extends into a hole cooperatively defined by the multiple arcuate plates. As the rotary shaft of the motor rotates, the annular spring applies a constraint force to the multiple arcuate plates so that a friction force is generated between the arcuate plates and the rotary shaft. However, the friction force generated in this construction changes little with the change of the rotational speed, which is disadvantageous in reducing the rotational inertia and the startup load torque during initial period of the startup, and hence cannot effectively address vibrations and startup failure during the motor startup.
Therefore, it is urgently desired to reduce the rotational inertia and startup load torque applied to the rotary shaft during the motor startup to avoid vibrations and startup failure.